The present invention relates to the art of drilling fluid processing, and more particularly to a system for degassing drilling mud. The present invention provides a centrifugal pump system for pumping gas laden drilling fluids while restricting the flow of gases removed from such liquids to conduits by which they are carried to safe disposal areas. Such a centrifugal pump system is desirable for the transfer of such gas laden fluids into degassing or deaerating vessels, or out of such vessels during periods of incomplete degassing.
In drilling a well for oil, gas and the like, the drilling bit is supported in the well bore by tubing. The tubing is hollow pipe composed of a plurality of individual lengths of pipe connected together. The tubing carries drilling fluid in its interior down to and through the drilling bit. The drilling fluid at the bottom of the well bore passes upwardly in the annulus between the exterior surface of the tubing and the interior surface of the well bore to the surface of the earth and then through a return pipe to storage pits on the surface of the earth commonly referred to as mud pits.
The drilling mud is ordinarily an aqueous suspension of solid matter generally containing minerals such as bentonite and barite. The drilling mud lubricates and cools the drill bit and serves as a carrier to withdraw drill cuttings and debris from the well for disposal. The drilling mud also provides a pressure seal in the well bore to prevent the escape of gases from the well. The pressure exerted by the column of drilling mud normally is greater than the pressure which may be released upon encountering gas pockets as the well is drilled. The column of drilling mud counteracts gas pressure and prevents blowouts but very often becomes contaminated with the gases encountered during the drilling operation.
Under many circumstances it is desirable and in fact often absolutely essential that the gases be removed from the drilling mud and transmitted to a disposal area. Since it is economically unfeasible to discard the contaminated drilling mud and because of the danger of the gases in the mud being released into the atmosphere in large quantities creating dangerous conditions at the drill site, it is necessary to process the mud to remove the gases and recirculate the degassed drilling mud through the borehole. The contaminating gases may be poisonous or highly explosive and the release of such gases into the atmosphere would present a substantial risk to personnel in the drilling area. The presence of gases in the drilling mud decreases its weight and affects its viscosity often rendering it unsuitable for recirculation through the borehole. When gases are contained in the drilling mud being circulated through the borehole, it increases the danger of a blowout in the well.
A "Notice to Lessees and Operators of Federal Oil and Gas Leases in the Outer Continental Shelf, Gulf of Mexico Area" was released May 7, 1974 by the United States Department of the Interior Geological Survey, Gulf of Mexico Area, relating to hydrogen sulfide in drilling operations. The notice outlines requirements for drilling operations when there is a possibility or probability of penetrating reservoirs known or expected to contain hydrogen sulfide. Section 3. f. provides that "drilling mud containing H.sub.2 S gas shall be degassed at the optimum location for the particular rig configuration employed. The gases so removed shall be piped into a closed flare system and burned at a suitable remote stack."
The prior art shows examples of systems for the degasification of drilling mud, many of which utilize a vacuum tank and some sort of baffle arrangement which exposes the drilling mud to vacuum environment, thus causing the entrapped gas to be removed. This is only part of the task however for serious problems are encountered in the handling of drilling muds, particularly in evacuating the drilling mud from the vacuum tank to return it as degassed mud to the well head. Precise control of the rates at which drilling mud enters the vacuum tank, degassed mud leaves the vacuum tank, and gases are evacuated from the tank, is necessary in order to produce an acceptable product at the necessary rate.
Some systems of the prior art use a separate vacuum pump and often expose this apparatus to the possibility of ingestion of drilling muds, a situation which normally damages the pumping mechanism and, at the very least, forces the entire system to be shut down for cleaning. Prior art devices have also exhibited problems in matching the vacuum pulled on the vacuum tank with the flow requirements of the entire system, which may be continually changing.
Numerous efforts have previously been made to eliminate the use of a mud jet for effecting the flow of mud such as substituting a centrifugal pump for the mud jet. However, such previous efforts have not been entirely successful inasmuch as a pump tends to become air-locked when the supply of mud to the tank is insufficient, or when vortexing of mud in the tank permits air or gas to enter the pump or when air or gas is present in the mud for any reason. Even when self-priming centrifugal pumps are used, several minutes may elapse before the pump resumes effective pumping action and during that period the efficiency of the degassing operation in the tank is materially affected. Previous attempts to provide vapor-vented centrifugal pumps include that disclosed in U.S. Pat. No. 3,815,717 which is not practical in abrasive fluids such as drilling muds because of rapid wear on its seals. A design disclosed by U.S. Pat. No. 3,769,779 avoids the abrasion of seals but requires the freed gas to flow counter to the incoming fluid at one or more points.